Special counsel Robert Mueller ramped up his ongoing Russia investigation by impaneling a grand jury, indicating that he seeks to subpoena records and take witness testimony regarding President Donald Trump's alleged ties to the Kremlin.

The grand jury reportedly started weeks ago but news of their work became public on Thursday after two sources notified the Wall Street Journal. Attorneys confirmed to the Journal that a grand jury signaled much more intense scrutiny in the investigation.

“This is yet a further sign that there is a long-term, large-scale series of prosecutions being contemplated and being pursued by the special counsel,” University of Texas law professor Stephen I. Vladeck said.

Federal prosecutor Thomas Zeno portrayed the grand jury as "confirmation that this is a very vigorous investigation going on.” Zeno also pointed to Mueller's hiring of a New York attorney and former top Justice Department official Greg Andres.

“People like Greg Andres don’t leave private practice willy-nilly,” Zeno reportedly said. “The fact he is being added after couple of months shows how serious this is and that it could last a long time.”

Trump has repeatedly denied collusion with the Russian government and has called Mueller's investigation a “witch hunt.”